1. Field of the Invention
The present invention relates to improvements in intelligibility of voice signals produced by a microphone in high noise environments, and more particularly concerns background noise suppression and reshaping of certain voice component signals in the output of a microphone.
2. Description of Related Art
A class of microphones, termed first and second order gradient electret microphones, has been developed to provide noise canceling operation in noisy background environments. These microphones have been shown to produce excellent voice quality in ambient noise environments approaching 100 dBA. The microphone is often mounted as a part of a headset on a boom that extends from the side of the operator's head to the lips and is normally positioned about one inch from the lips. Intelligibility of the microphone output is high for normal voice levels when the microphone is used in this manner in high noise environments. However, in situations where the magnitude of background noise exceeds these levels, mainly in situations where background noise rises to the level of 110 to 120 dB, the operator must change the manner of use of the microphone in such a way as to seriously degrade speech intelligibility at the microphone output.
Electret gradient microphones derive their noise canceling qualities from the way they respond to audio signals emanating from areas in front of the microphone (near field response from the operator's lips) as compared to the way they respond to the signals reaching the microphone from the back or sides (far field or background noise response). The near field/far field response of an exemplary first order gradient electret microphone is illustrated in FIG. 1. The operator's voice enters the front of the microphone and follows the near response curve NR indicated in this figure whereas background noise enters the back and sides of the microphone, following the far response curve FR. The difference between these two responses, shown as a shaded area in FIG. 1, represents noise canceling ability of this class of microphones. The microphones provide excellent attenuation of noise (from 10 to 20 dB) at frequencies below about 1 kilohertz, but exhibit little noise cancellation above about 2.5 kilohertz.
When these microphones are used in very noisy environments, namely those in excess of 100 to 105 dBA, which are levels so high that the sound vibrations are literally felt as vibrations, the operator must use the microphone in a manner which quickly reduces voice quality and speed intelligibility of the microphone output. This mode of use in the very high noise environment has two major aspects which drastically affect intelligibility of the voice being conveyed in the communication channel to which the microphone output is fed. First the operator naturally raises his voice in an effort to compensate for this exceedingly noisy environment. Second, the operator places the microphone very close to his lips and may in fact place the microphone in contact with his lips. This is done in an effort to raise the signal level so that the speaker can hear his own side tones, which are fed back from the microphone to the earphones and through earplugs worn by the speaker in the very high noise environments.
This manner of use of the electret gradient microphone in very high noise environments produces two phenomena which degrade speech intelligibility. The combination of the raised voice level and close proximity of the microphone to the voice source increases the dynamic range of the microphone output beyond capabilities of current communication systems. Further, microphone output signals include certain higher amplitude components (termed "puff noise") that seriously degrade intelligibility of voice signal components in higher frequency ranges (above the noise canceling frequency range of the microphone). Puff noise is due to air flowing across the microphone element rather than to sounds generated in the creation of speech. Presently available intercommunication systems do not provide a dynamic range sufficient to handle signal levels produced by such electret microphones in very high noise environments without producing significant clipping distortion of the voice signals. Digital signal processing techniques have been tried to increase intelligibility, but these have required massive processing capability and are currently limited to the laboratory. Further, such systems are much less cost effective in producing intelligibility improvements.
Various methods have been devised for suppressing background noise in other types of microphones and other systems, including noise suppressing circuitry shown in U.S. Pat. No. 4,461,025 to Franklin, and U.S. Pat. No. 4,630,302 to Kryter, but these are operative primarily to eliminate that portion of the noise which inherently is eliminated by the operation of a first or second order gradient electret microphone itself. Such systems are not operative to suppress the puff noise contained in the output signal of a first or second order gradient electret microphone when used in very high noise environments. The prior art will not operate to attenuate or extract intelligence from puff noise in the output of such microphone due to the unique method of use of the microphone that occurs in the very high noise environment.
Accordingly, it is an object of the present invention to increase intelligibility of voice signals in high noise environments by reducing or eliminating detrimental side effects of puff noise associated with electret gradient microphones and to use such information to increase discrimination of speech components.